Aluminum nitride (AlN) and AlN epitaxial films grown on bulk AlN substrates have several uses due to the unique properties of the AlN material system—particularly its very high band gap energy. In some instances, the substrate may need to undergo extreme thinning, such as to less than 20 microns, or be completely removed. One such example consists of a UV-C VECSEL (vertical external cavity surface emitter laser) device emitting near 240 nm. The term ‘bulk AlN’ refers to AlN single crystalline AlN boules that are sectioned into wafers on which epitaxial material is grown.
Vertical lasers rely on light traversing the laser cavity multiple times. This requires that the reflectors be highly efficient, and that the cavity have very low absorption. At UV-C wavelengths, bulk AlN substrates exhibit absorption loss to the degree that even a micron of substrate material in the cavity would inhibit the gain necessary for successful lasing. Removing the substrate, leaving only higher quality epitaxial material in the cavity, becomes an essential process and device structure. Thin film ergodic LED devices represent another example where light must traverse a low loss cavity multiple times. Substrate removal can also provide benefits to electronic devices. In addition, substrate-removed devices may be fused or bonded onto substrates with superior properties to the one upon which they were formed. Substrate thinning or complete removal and subsequent bonding or fusion can enable devices that require a thin channel in contact with a contrasting material, such as semiconductor on insulator (SOI) and radio frequency devices.
For class III-V semiconductor devices substrate removal is typically employed for devices grown heteroepitaxially on non-native substrates. In this case, the substrate removal processes typically exploit material differences that exist between the substrate and epitaxial layers to remove the substrate while maintaining the device layers intact.
For example, a laser lift-off process can remove gallium nitride (GaN) blue LED device layers grown on sapphire. The laser employs a wavelength transparent in sapphire but absorbed by GaN. Selective ablation at the substrate/epitaxial layer interface causes the epitaxial layer to separate from the substrate. Other examples include AlGaInP LED device layers grown on red light absorbing gallium arsenide (GaAs) substrates. The substrate is removed by a chemically selective wet etch that attacks the substrate but not the device layers. Another example involves separating device layers by photo-electrochemical (PEC) undercut etching of a strategically located n-type epitaxial material layer.